Co-axial resistive load



y 1959 B. D. FREDERICO I 2,894,219

CO-AXIAL RESISTIVE LOAD Filed March 18, 1955 INVENTOR. 6205s y a Ream/coATMRNEYS United Sttes Patent CO-AXIAL RESISTIVE LOAD Blossy D.Frederico, Cleveland, Ohio Application March 18, 1955, Serial No.495,129

4 Claims. or. 333-22 This invention relates to the art of electricalco-axial high frequency resistive loads employing compound dielectrics.

There have been numerous attempts to design a noninductive terminationfor use as a dummy load for dissipating the radio frequency energy oftransmitters. At first there was a non-inductive wire-wound resistorunit which however proved unsatisfactory as the frequency rateprogressed into the higher range; such a unit did not act as a pureresistance but tended to become more and more reactive. Then came theresistors of the inked-on film type and the deposited-carbon type whichsatisfied the radio field temporarily but, as the rate of frequencymoved higher and higher, neither did they behave as pure units. Tuningmethods and the like were applied to these later units to cancel out thereactive portion of their characteristics at the higher frequencies butthis method had many disadvantages, notably that of being a fixedfrequency device which had to be readjusted for every change infrequency.

There were attempts to overcome these disadvantages, as for instance inthe US. patent to Von Radinger No. 2,273,547, February 17, 1942, whichused the film-type of resistor within an exponentially contoured hornfitted co-axially about the resistor element and which, to a limitedextent, was a decided step towards the designing of terminations asdummy loads in the co-axial field of radio transmission and reception.

In the first attempts of such horn-type of device, air was used as thedielectric medium but, with the everincreasing demand for units capableof dissipating greater and greater radio-frequency power, these deviceshad to be re-designed. Then there was attempted the same de sign withforced air-cooling but this did not greatly multiply the power-handlingcapabilities of the dummy loads" and it was at this stage that a liquidwas adopted to replace the air dielectric. The advantage of the liquidwas utilized to increase the power-rating of the dummy (antenna) load,and this too was a notable step towards a more eflicient heat-exchangingdevice. Then later there was added to such a device a secondary orauxiliary device to cool the dielectric liquid, which in turn would[reduce the resistor temperature and thereby increase the power-handlingcapabilities of the termination (dummy load). This method consisted ofcoiled copper tubing with its coil immersed in the dielectric liquidoutside of the horn enclosure. It is known by those versed in the designof horns for co-axial terminations that foreign objects and materials,not dictated by the design parameters, must be kept out of thedielectric space between the resistor and the horn enclosure; and it isalso well known that water has tremendous cooling characteristics and,if used to its greatest advantage, may excel all other liquids in manycooling applications.

The prior design of heat exchanger, just referred to, depended upon thenatural phenomenon of convection currents; that is, the dielectricliquid was set in motion by the rise in temperature of the dissipatingelement (resistor) of the horn assembly, the convection currents being afunction of a change in fluid density that is produced by the rise inits temperature. In an attempt to further increase the power-rating ofthis type of dummy 12,894,219 Patented July 7, 1959 ice load, there wasthen added a pump for a more rapid cooling of the hot dielectric liquidwhich was pumped through the horn assembly and out into a tank where'the water-cooling coils Were located to cool the hot liquid, the cooleddielectric liquid being then returned to the horn.

It is the design of device, just referred to, that is cur-' rently usedas a dummy load in the co-axial field to absorb the electrical energy,and the demand for higher and higher power-handling units has caused thedesigners to resort to larger resistor units in their horn designs; butthis step is in the wrong direction for broad (frequency) bandapplications because of the attendant wave-guide effects. In view of thefact that the larger the diameter of the resistor, the larger the insidediameter of the horn must be, it follows that as the radio frequency isincreased, the dummy load more likely will not function in the co-axialsense but will be excited more naturally into a wave-guide sense.However, it will be shown that this limitation can be overcome, evenwith resistors having as physically large size as are being presentlyused. The larger resistor in the present-day design does aid inincreasing the power-handling ability of the unit but, as pointed out,it tends to reducemore and more its ability to be used at higher. andhigher frequencies in which of course the radio field is alsointerested. There is an important application for high-power loads atthe higher frequency bands and perhaps the obvious solution would havebeen to use water directly as the dielectric field but this was notfeasible with the existing facts. Water has a very high dielectricconstant and this would have figured directly in the former designequation of the abovenoted Von Radinger type of device which would haveresulted in an impractical design limited to a narrow hand because ofwave-guide effects; and also it would have been so large in its physicalproportions that hoisting means would have been required to move itabout. The impractica' bility of such a design is further made apparentby noting the dielectric constant of water, which is 81 as compared with1 for air. The physical proportions of such a de-. sign would bemultiplied beyond practical limits if water alone were substituted forair as the dielectric medium in the design of a co-axial termination(dummy antenna) in accordance with the teachings of the above patent toVon Radinger. However it will be shown herein that there is a uniquesolution to the foregoing limitations of water or air alone as adielectric medium in co-axial terminations; and I will show that, bydesigning an instrument (dummy load) with the small geometryadvantagesthat are characteristic of air as a dielectric and thepower-handling advantages of water as a dielectric, such a multiple orcompound dielectric form of device would be a distinct and notableimprovement over the presentday single dielectric form of device.

Accordingly, it is an object of my present invention to relieve theabove-noted problems and complications in a non-inductive termination;and, more definitely, to design a resistive load in which there iswidened frequency range and a high-power rating; to devise a practicallyreflectionless resistive load that is capable of dissipating largeamounts of power; to avoid the necessity of employing auxiliary means ofadjustment or tuning in such a device; to avoid the wave-guide efiect insuch a device; and to provide such a device that is within reasonablephysical proportions. i i

More specifically, my present invention is directed to theaccomplishment of the above-noted objects by designing amultiple-dielectric device in which there is included a logarithmichornand in which there are utilized. the small geometry advantages ofair as a dielectric and the power-handling advantages of water as adielectric,

and in which the contour of the horn is arrived atby means of anestablished equation for multiple dielectrics which I have discovered tobe applicable for the accomplishment of this part of my presentcombination.

That is to say, .1 have discovered the applicability of a mathematicalequation or relationship that makes possible a design using more thanone dielectric medium within a horn form of outer enclosure. The horn inmy present device takes on a more complex contour than that of the aboveVon Radinger patent because of the ex panded equation that is employedinstead of his simple relationship. It maybe said that my expandedequation is multiple in its function and may be exploited to producedevices that have high-power ratings, broadband operation, simplicity ofconstruction, low-cost maintenance, reduced cost of fabrication, less,bulk, etc; and these briefly constitute the objects of my presentinvention.

Other objects will appear from the following description and claims whenconsidered together with the accompanying drawing.

Fig. 1 is a perspective view of my present form of device;

Fig. 2 is a longitudinal sectional view thereof;

Fig. 3 is a transverse sectional view taken on line 33 of Fig. 2.

It is to be understood that the present form of disclosure is merely forthe purpose of illustration and that there might be devised variousmodifications thereof without departing from the spirit of my inventionas herein set forth and claimed.

The inventive thought in my present form of device is to design abroad-band co-aXial (dummy load) termination with direct water-coolingof the resistor element .of the horn assembly. While other designershave used direct water-cooling to increase thepoWer-rating, such deviceshave been narrow frequency-band units that require adjustments such astuning, etc. None of the prior devices, so far as I am aware, has usedthe known mathematical equation for fashioning an enclosure that makesit possible to broad-band the device and also to get a high-powerrating. By using the mathematical parameters derived, a horn enclosurecan be designed with more than one dielectric material within theenclosure that .coaxially surrounds the implanted resistor element, andmy present form of device may be called a co-axial resistive terminationof multiple or compound dielectrics with means of direct fluid-cooling.

If air is used as the complete dielectric material, it gives a smallconfiguration to such a, horn enclosure and a wide-band frequency rangewhile water or other suitable =liquids, if used as the completedielectric material, makes for a large configuration but high-powerrating and a limited frequency range. Now, my solution of the situationis to provide a compound dielectric with a cross section of water for ahigh-power rating and a larger cross section of air fora wide frequencyrange; and I have discovered that the use of mathematical parameters(see the equation below) for compound dielectrics in co-axial systems,makes such a design possible.

It 'willbe noted here that my present form of invention includes a thirddielectric material in addition to the air and water; namely, adielectric tubing to carry-the liquid (water) flow along the outside ofthe resistor element. The three dielectric materials are proportioned togive the maximum advantages; and the known dimensions of some of them,electrical and physical, are substituted in the equation in order toarrive at the size and hence the contour of the horn enclosure. It isimportant to state here that my final design will be a practicallyreflectionless (low voltage standing wave ratio) resistive co-axial load.capable of handling large amounts of electrical energy over awide-frequency range without using auxiliary tuning mechanism oradjustments. This dummy load can be used with calorimetry or withsuitable current or voltage devices .to measure electrical energy.

My present assembly in general consists of the electrical ho'llow formof resistance 1 which is surrounded,

by the co-axially spaced cylindrical shell 2 of dielectric material toprovide a passage for water or other suitable liquid, and the co-axiallyarranged electro-conductive horn 3 which provides an enclosure for theair as another dielectric. The :shell 2 may consist "of plastic, Pyrex,

. glass or other suitable dielectric for piping a liquid. The

cooling liquid (water for instance) enters through the hollow center ofthe resistor element 1 at the .end ..corresponding to the small end ofthe horn 3 and flows along the length of the same in intimate contacttherewith and out through vent holes 4a in the electro-conductive collar4,:suitably applied to the end of the resistor -.element 1,

and alongthe inside of the dielectric tube 2 andais ex pelled from theopenings 3a in section 3a of the horn 3. The direction of flow of thisliquid may be :reversed.

. The back seal comprises flanges 5 and 6 .of the horneccsistor element1 at the short-.circuited end, .as indicated in the present drawing.

The inside diameter of the horn 3 diminishes along its longitudinal axisand .the .contourof this horn is a function of the following equationwhich dictates the transverse diameters taken at incrementaldistancesalong the longitudinal axis of the assembly:

?l t T b logloglog logel 62 es in which Z is .the impedance at any planetransverse to the axis of the co-axial conductive horn .3. and:the:resistor element 1,

log-g is the logarithm of the ratio between the diameters of the outerhorn 3 and the resistance element 1,

log-g is the logarithm of the ratio between the outer diameter of theliquid dielectric body and the diameter of the re sistance element 1,

log-

is the logarithm of the ratio between the outer diameters of thedielectric body 2 and the liquid dielectric body,

log?

is the'logarithm of the ratio between the diameter of the outer horn .3and the outer diameter of the dielectric body 2, and e 2 and e are thecoeificients of the three dielectric bodies-namely, the water, the body2 and the air within the horn 3, respectively.

These incremental distances are stepped along the axis of the resistorelement 1 and the value of Z will vary accordingly at each transverseplane looking into the axis of the structure towards the end of the horn.that idiminishes to the .point of intimate contact with the resistorelement 1. As may be understood, the electrical in put is ,at the end ofthe resistance element 1 correspond ing to the larger end of the horn 3and an outer electrical conductor will be connected-to the largerend ofthe born 3 which will be provided with suitable means for this purpose.:Such connector means may be of the tapered form or of the morepractical radio-frequency stepped type (oflset inner to outerconstruction).

The characteristic electrical impedance at a transverse plane lookingback towards the diminishing end of the horn enclosure (i.e. Z will haveits maximum value (ohms) at the large or input end at plane A--A in Fig.1, this being what may be called the start of the resistance materialwhich may be in the form of deposited carbon or metal resistive film;and the value of Z diminishes therefrom to zero at the plane of intimatecontact of the horn section 3a with the resistor element 1. Theresistive film of the resistor element 1 ends at the plane BB and, formechanical considerations, the electrical contact is carried a shortdistance beyond this point.

In solving the above equation for the diameter of the born 3 at anygiven point along its longitudinal axis, we may assume the radial extentof the clearance or space for the flow of the liquid about theresistance element 1 and We may assume also the thickness of thedielectric shell 2. Then, knowing the diameter and length of theresistor element 1 and also its direct-current resistance per unitlength, we may assume the resistance of the resistor element 1 whichwill be adopted as the impedance (ohms) at any given distance from thepoint where horn 3 joints shell 2 (plane BB); and by substituting theseknown values in the above equation, there may be found the diameter ofhorn 3 at the point corresponding to that plane selected along theresistor 1. Thus the length and ohmic value of the resistor element 1may determine the plane at which the equation is applied in eachinstance; and these planes should be selected at small intervals alongthe longitudinal axis of the resistor in order to obtain the best designfor the horn 3. When these successive diameters for the horn have beendetermined, their outer ends will be connected and the form of the hornsurface developed therefrom.

Another detail may be added to the design which will further improve theelectrical performance at high frequencies; that is, the provision oflongitudinal electroconductive appendages 9 within and as integral partsof or in contact with the horn 3 to further discourage waveguideeffects. These tongues 9 need not run the entire length of the horn butonly a short distance from the input or larger end thereof. In thepresent illustration there are shown four such appendages 9 spaced atninety degrees about the longitudinal axis of the horn and that may beformed as integral parts thereof by the stamping process and they mayextend for about one-fourth of the length of the horn 3, for instance.These appendages are so placed as not to interfere with the co-axialmode of operation but are so positioned as to co-operate in obtaininganti-wave-guide transmission.

Some of the details herein shown have the advantage of beingself-compensating electrically; and it is to be understood that themechanical or physical arrangement shown in the present drawing need notbe the only way in which this combined unit may be fabricated, as aboveindicated.

There might appear to be an electrical upset in the horn enclosure atthe point of the flanges 5 and 6, which is true physically but notelectrically because the high capacitauce represented by the indicatedarrangement of the metallic faces of the flanges 5 and 6 functions toshunt the upset at higher frequencies, while at lower frequencieselectrical continuity is sufficiently satisfactory not to produceundesirable electrical reflections. The operation of my present co-axialresistive termination is believed to be altogether dependable in boththeory and practice.

My present invention possesses several marked advantages, some of whichwill be here briefly noted while still others will no doubt readilysuggest themselves to those who are familiar with the art to which thisinvention relates. My present design simplifies the mechanical andelectrical problems above referred to; the frequency range is widened byuse of the multiple or compound dielectric scheme is designing myresistive load which is practically reflectionless and is capable ofdissipating large amounts of power. By providing a comparatively thincross sectional flow of water, I have relieved the previous problem ofthe high dielectric constant of such a liquid; and I have discovered theapplicability of a mathematical equation by which I am enabled to designa practical form of device. To repeat, I have devised a practicallyreflectionless resistive co-axial load that is capable of handling largeamounts of electrical energy over a wide-frequency range and withouthaving to resort to auxiliary tuning mechanism or adjustments.

In referring to the diameter of the logarithmic horn throughout thepresent description and claims, it is to be understood that I have inmind the inner diameter thereof, as mentioned in column 2, lines 14 to19.

What I claim is:

1. An electrical resistor comprising an element, that is predominantlyelectro-resistive, and co-axially arranged separate and diametricallyspaced tubular bodies surrounding said electro-resistive element andadapted to contain fluids of different dielectric coeificients,respectively, the outermost of said bodies being electro-conductive andbeing in the form of a logarithmic horn which is a function of saidmultiple dielectrics, and said horn having unitary means extendinginwardly therefrom and being of an electro-conductive nature.

2. The same structure as recited in claim 1 hereof and in which saidunitary means is in the form of a plurality of integralelectro-conductive projections extending inwardly from said horn inspaced relation to each other thereabout.

3. An electrical resistor comprising a hollow element that ispredominantly electro-resistive, a co-axially arranged cylindrical bodyof dielectric material arranged in spaced relation about saidelectro-resistive element, said electro-resistive element being open atone end and having means of communication at its other end with thespace between the same and said cylindrical body, means for mountingsaid cylindrical body at its two ends upon said electro-resistiveelement, and an electro-conductive logarithmic horn mounted at its smallend co-axially upon said cylindrical body and defining an air spacewithin said horn, the larger end portion of said horn being providedwith unitary inwardly extending projections of electroconductive naturespaced thereabout.

4. An absorber of electrical energy comprising an element, that ispredominantly electro-resistive, and coaxially arranged separate anddiametrically spaced tubular bodies surrounding said electro-resistiveelement and adapted to contain two different fluids, respectively, oneof which fluids is adapted predominantly to serve as a dielectric andthe other of said fluids is adapted predominantly to serve as a thermalcarrier, the outermost of said tubular bodies being electro-conductiveand being in the form of a logarithmic horn whose specific shape is afunction of said fluids, said electro-resistive element being of hollowform and open at one end thereof and having means of communication atthe other end thereof with the corresponding end of the inner one ofsaid tubular bodies, and the inner one of said tubular bodies having anopening at the other end thereof so as to permit passage of a fluidbetween said electro-resistive element and said inner tubular body.

References Cited in the file of this patent UNITED STATES PATENTS2,399,930 Keister May 7, 1946 2,463,428 Rieke Mar. 1, 1949 2,561,184Dehn July 17, 1951 FOREIGN PATENTS 695,166 Great Britain Aug. 5, 1953

